Synthesis And Applications Of Carbon Based Phosphorescence Materials

Phosphorescence is a slow luminescence process,which refers to the excited triplet excitons back to ground state with radiative recombination.Because of the de excitation process of phosphorescence is spin forbidden,the process is usually very slow,and the luminescence lifetime is usually on the order of microseconds to seconds.Due to long lifetime,large Stokes shift and high signal-to-noise ratio,phosphorescence materials have been widely used in the fields of optoelectronic devices,chemical detection,information encryption,information storage and bioimaging.To date,efficient phosphorescence materials are mainly limited to organic phosphorescence materials,rare earth based phosphorescence materials and noble metal based phosphorescence materials.However,the above materials usually suffered from complicated preparation process,high cost,poor stability and high toxicity.Therefore,it is urgent to develop a new phosphorescence material with low toxicity and easy preparation.As a new class of luminescence nano materials,carbon based nano materials(including carbon based organic materials,carbon dots,carbon nanodots,carbon polymer dots,carbon quantum dots,etc.)have attracted extensive attention because of their high luminescence efficiency,good biocompatibility,easy preparation and low cost.It has a good prospect to explore the carbon based nano materials with different phosphorescence emission properties,and then develop their applications in different fields.Almost all the reported phosphorescence carbon based nano materials are in solid form,and their phosphorescence will be quenched in aqueous solution,which greatly limits its application.In this work,the study is focused on the problems of poor solubility,phosphorescence quench in aqueous solution,low luminescence efficiency and single emission wavelength of phosphorescence materials.Due to the easily adjusted structure and easy preparation,water-induced ultralong room temperature phosphorescence in organic microrods by constructing hydrogen-bonded networks can be achieved.On this basis,green and red/near infrared phosphorescence emission of carbon nanodots in aqueous solution have been achieved through triplet exciton protection strategy and F(?)rster energy transfer strategy.The quantum yield and lifetime are the best values ever reported for water-soluble phosphorescent nanoparticles.With these efficient phosphorescence carbon base nano materials,multi-level information encryption and information storage are realized;Phosphorescence carbon nanodots are applied in multiplexing technology,and spatiotemporal overlapping information can be separated by water;In vivo/vitro afterglow imaging is also been demonstrated by using phosphorescence carbon nanodots.The results include the following aspects:1.Water-induced phosphorescence by constructing hydrogen-bonded networksBased on the confinement effect of hydrogen bond network,and selecting optimal precursors with large electronegativity and small atomic radius,organic microrod materials with water induced phosphorescence emission property were synthesized.The lifetime is up to 1.64 s,which is the longest lifetime of organic phosphorescence materials in aqueous solution;Through structure characterization,it is found that hydrogen bond is formed between water molecules and material molecules after the addition of water,and the hydrogen bond network can effectively limit the movement of molecules and reduce the nonradiative transition caused by molecular rotation,so the triplet exciton can be stabilized;The spin orbit coupling(ξ)between the singlet and triplet states can be increased and energy gap can be decreased after adding water,which is the reason for the enhancement of phosphorescence emission.2.Achieving phosphorescence emission of carbon nanodots in aqueous solutionBased on the reasons for the quenching of phosphorescene of organic materials in aqueous solution and the the preliminary exploration results of realizing the water induced phosphorescence emission of organic microrods,the key to realize the phosphorescence emission of carbon nanodots in aqueous solution is as follows:(1)the doping of heteroatoms(N,P elements)can enhance intersystem crossing(ISC)from singlet(S1)to triplet(T1)excited states and promotes the generation of triplet excitons;(2)the electron transfer path between carbon nanodots and dissolved oxygen in aqueous solution can be cut off by hydrophilic silica,which can not only ensure the solubility of carbon nanodots,but also stabilize the triplet exciton of carbon nanodots and realize phosphorescence emission in aqueous solution;(3)Phosphorescence can be enhanced by reducing the nonradiative transition through the confinement effect of external matrix;Based on the above analysis,phosphoric acid and ethylenediamine containing N and P atoms were selected as precursors,and solid phosphorescence carbon nanodots(CNDs)with a lifetime of 1.33 s were synthesized by microwave method;Triplet protection strategy was proposed.Hydrophilic silica was selected as the matrix to coat solid phosphorescence CNDs,and water-soluble phosphorescence carbon nanodots(WSP-CNDs@silica)were constructed,the phosphorescence emission of CNDs in aqueous solution was realized.The phosphorescence lifetime of WSP-CNDs@silica was 1.86 s and the phosphorescence quantum yield was 11.6%,both of which were the highest values of phosphorescence materials in aqueous solution;The phosphorescence emission mechanism of WSP-CNDs@silica in aqueous solution is verified by experiments:① covalent bond is formed between silica and CNDs,which can effectively reduce the non-radiative transition caused by vibration and rotation of CNDs and stabilize its triplet exciton;② The isolation of silica can cut off the electron transfer between CNDs and dissolved oxygen in aqueous solution,and the triplet exciton can be protected,the ultralong phosphorescence emission of CNDs in aqueous solution can be realized.3.Forster resonant energy transfer for achieving red/near infrared phosphorescence of carbon nanodots in aqueous solutionBased on triple state protection strategy and Forster energy transfer strategy,green phosphorescence CNDs were selected as the energy donor and red/near infrared dye rhodamine(RhB)as the energy receptor to construct the donor-receptor pair(CNDsRhB@silica).Red/near infrared phosphorescence emission of CNDs-RhB@silica can be achieved in aqueous solution.The Forster energy transfer efficiency can reach 99.2%by adjusting the concentration of receptor.In addition,the relationship between the F(?)rster energy transfer efficiency and distances between CNDs and RhB is in line with F(?)rster model,and the Forster distance is 1.74 nm;The ultralong lifetime and high phosphorescence efficiency of CNDs-RhB@silica due to the excellent energy matching between phosphorescence CNDs and RhB.The lifetime of the CNDsRhB@silica nanocomposites can reach 0.91 s with phosphorescence efficiency of 3.56%,both of which are the best values ever reported for water-soluble red afterglow nanoparticles4.Exploring the applications based on the carbon based phosphorescence materialsCarbon nanodots and organic microrods with good phosphorescence properties prepared in this paper are used to explore their related applications.Firstly,the optical and thermal stability of organic microrods are investigated,and then the advanced information encryption and information storage based on the organic microrods have been demonstrated;Secondly,the cytotoxicity and optical stability of WSPCNDs@silica were evaluated.The experiment indicates that the WSP-CNDs@silica has low cytotoxicity and high optical stability.Flow cytometry assays show that WSPCNDs@silica can bind to living cells.Then,the in vivo afterglow imaging was carried using the WSP-CNDs@silica as afterglow agent;The cytotoxicity and optical stability of CNDs-RhB@silica were evaluated.In addition,its deep tissue imaging ability was explored and its tissue penetration depth was 5 mm,the signal-to-noise ratio in vivo imaging was as high as 80.1;The lifetime of the CNDs can be manipulated from nanosecond level to second level by introducing water.Time division duplexing based on the CNDs and CNDs@silica with distinct lifetimes is realized and spatio-temporal overlapping information is thus resolved.